This invention relates to liquid crystal compounds and, more particularly, novel tolan derivatives, liquid crystal compositions containing the tolan derivatives and liquid crystal display devices including the compositions.
Liquid crystal display devices utilize the electro-optical properties of liquid crystal materials. Liquid crystal materials have three phases which are known as the nematic phase, the cholesteric phase and the smectic phase. The display system most widely used at present is the twisted nematic type ("TN") which uses the nematic phase of liquid crystal materials.
The advantages of liquid crystal display devices is that they can be compact and thin. Additionally, liquid crystal display devices are driven by low voltage and so require only a small amount of electric power. Liquid crystal display devices are also light-receiving elements and so cause limited eye fatigue to the user even after extended viewing. Because of these advantages, liquid crystal display devices have been used widely in watches, desk-top calculators, audio devices, measuring instruments, automobile dashboards and the like. More recently, liquid crystal materials have been used in displays containing numerous picture elements such as black-and-white or color pocket televisions. Similarly, liquid crystal display devices have begun to replace CRTs in personal computers and word processors.
Although the required characteristics for liquid crystal material varies depending upon in which type of device the material is to be used, all liquid crystal materials must have the following characteristics:
1. the material must be transparent as well as electrically and chemically stable even when exposed to heat and light;
2. the material must perform in a wide range of temperatures;
3. the electro-optical response speed of the material must be rapid;
4. the voltage required to drive the material must be low;
5. the material must need only a small incremental change in voltage to cause a sharp change in the material between the activated and non-activated states, and the temperature dependence of the threshold voltage value ("V.sub.th ") must be small; and
6. the material must provide a wide visual angle range at which the device may be viewed.
A number of known liquid crystals satisfy the first characteristic. However, no single liquid crystal compound is known which, by itself, satisfies the second through sixth characteristics. Thus, in order to have all six characteristics, a liquid crystal composition must contain several kinds of nematic liquid crystal compounds or non-liquid crystal compounds, each compound possessing one or more of the desired characteristics.
To satisfy the second characteristic, at least two liquid crystal compounds are used. The first compound has a relatively low molecular weight and has a crystal-nematic transition point (the "C-N"point) or melting point at or about room temperature. The second compound has a relatively high molecular weight with a transition temperature between phases (the "N-I" point) of 200.degree. C. or more. The N-I point of the second compound is ideally as high as possible to provide the maximum temperature range at which the liquid crystal composition may perform.
In order to satisfy the third characteristic, it is necessary to decrease the cell gap (designated "d") in the liquid crystal compound to increase the response speed (designated "r") because of the following relationship: EQU .tau..alpha..eta./d
whereby .eta. is the viscosity index. However, to prevent generation of interference fringes on the surface of cells which deteriorate the appearance of cells, the value .DELTA.n.multidot.d should be a constant value. Therefore, the liquid crystal compound should have a large .DELTA.n so that the value of d can be small to increase the response speed.
To satisfy the fourth characteristic, the liquid crystal compound should require a low threshold voltage. The following is the relation between V.sub.th, the elastic constant K and .DELTA..epsilon.: EQU V.sub.th .alpha.(K/.DELTA..epsilon.).sup.1/2
Thus, in order to decrease V.sub.th, the liquid crystal compound should have a large .DELTA..epsilon. and a small K.
Accordingly, it is desirable to provide a liquid crystal compound to be used in a liquid crystal composition which not only has the first characteristic listed above, but which also satisfies the upper temperature limit of the second characteristic as well as having the third and fourth characteristics.